This invention relates to a digital-to-analog (D/A) conversion circuit constructed a plurality of D/A converters.
FIGS. 1, 2 and 3 show three conventional converters for converting a digital signal to an analog signal.
The D/A converter as shown in FIG. 1 is called a weighted resistor type D/A converter. Between constant voltage source 10 and summing amplifier 12 are connected an n number of weighted parallel resistors having resistive values 2R, 4R, 8R, 16R . . . 2.sup.n R corresponding to bits B1, B2, B3, B4 . . . Bn of input digital data. Between the parallel array of the weighted resistors and constant voltage source 10, switches SW1, SW2, SW3, SW4 . . . SWn are connected in series with the weighted resistors 2R, 4R, 8R, 16R . . . 2.sup.n R, respectively. Switches SW1, SW2, SW3, SW4 . . . SWn, upon receipt of the corresponding bits "1" of the input data, allow the corresponding resistors to be connected to constant voltage source 10 and, upon receipt of the corresponding bits "0" of the input data, allow the corresponding resistors to be connected to a common potential through a break of the connection between the resistors and constant voltage source 10.
The D/A converter as shown in FIG. 2 is called a ladder resistor type D/A converter. Between constant voltage source 10 and summing amplifier 12 are connected an n number of parallel resistors having the same resistive values 2R, 2R, 2R, . . . , 2R corresponding to respective bits B1, B2, B3, . . . Bn of input data, and between the parallel resistors are connected an n-1 number of resistors R having the same resistive values R. In this way, a the R-2R type ladder circuit is formed in which switches SW1, SW2, SW3, . . . SWn are connected in series with the corresponding resistors (2R, 2R, 2R, . . . , 2R). Switches SW1, SW2, SW3 . . . SWn, upon receipt of the corresponding bits "1" of the input data, allow the corresponding resistors to be connected to constant voltage source 10 and, upon receipt of the corresponding bits "0" of the input data, allow the corresponding resistors to be coupled to a common potential through a break of the connection between the resistors and constant voltage source 10. Resistors 2R are connected between the common potential level and a junction connected between the resistors (2R) corresponding to an LSB (least significant bit) and the adjacent resistor R.
A D/A converter as shown in FIG. 3 is called "a weighted constant current type D/A converter". Between power source 14 and summing amplifier 12 is connected a parallel array of weighted constant current sources I/2, I/4, I/8, I/16 . . . I/2.sup.n corresponding to respective bits B1, B2, B3, B4 of input data with switches SW1, SW2, SW3 . . . SWn connected in series with constant current sources I/2, I/4, I8, I/l6 . . . I/2.sup.n.
In these D/A converters, shown in FIGS. 1 to 3, when any of these switches are turned ON and receive the bits "1" of the input data, then an analog signal corresponding to the bit "1" positions of the input data, that is, an analog signal corresponding to the input data, is output as a voltage V.sub.O.
In these conventional D/A converters, the accuracy of the output analog signal with respect to the digital input signal is influenced by the accuracy of the resistors or the constant current sources. In order to obtain an analog output signal of high precision, it is necessary to employ resistors of uniform accuracy. However, it is difficult to prepare a large number of resistors of identical precision and, even if it could be attained, the manufacturing cost would become high. It is also difficult to construct a large number of constant current supplies of uniform accuracy.
Thus, in the conventional D/A converter there is a tradeoff between the high precision of the component parts involved and a high manufacturing cost.